2D Flow Area Connection Issues

A flow area connection can be used to model a hydraulic structure between two adjacent storage areas, a storage area and 2D flow area, two adjacent 2D flow areas or within a single 2D flow area. The flow area connection within a single 2D flow area represents how an internal hydraulic structure allows flow to travel from one set of cells to another set of cells. For example, it can be used to model a roadway crossing in the middle of the 2D flow area, or similarly, represent a dam structure within the 2D flow area.

dam structure with a culvert passing through it

Example of a 2D flow area connection representing a dam structure with a culvert passing through it

Mesh Alignment Issues

For an internal 2D flow area connection, there should be a breakline that coincides with the 2D flow area connection alignment. Breaklines are used to force alignment of the computational cell faces along barriers or other features that can significantly affect the 2D flow. In GeoHECRAS, the software automatically makes certain that the 2D mesh aligns with the connection element by automatically inserting a breakline coinciding with the 2D flow area connection alignment. This insures that cell faces are aligned perpendicular to the flow going over the structure. This prevents flow from leaking through a structure if the 2D cell is large enough that it straddles both the upstream and downstream side of the structure.

2D cell straddling element

Example of 2D cell straddling element, causing flow to leak through the roadway crossing

 cell faces with the structure

By aligning the cell faces with the structure, the leaking of flow is eliminated

Partial Cell Face Coverage

If the 2D connection element starts or ends on a partial 2D flow area cell face, then the HEC-RAS preprocessor will report the following error message:

There is an error with the model data. The weir station/elevation extends too far beyond the last face point (face intersection).

This is shown below.
2D connection Error Message

This situation occurs when the 2D connection element does not cover a majority of the 2D cell face. This can cause confusion for the computational engine, since it cannot determine if the flow should be through the 2D connection element or between cells faces. This is shown below.

This 2D flow area connection will fail to run

This 2D flow area connection will fail to run

Correcting the 2D Flow Area Connection Issue

By extending (or shortening) the 2D connection so that it occupies all (or most) of the cell face, the problem is resolved.

This 2D flow area connection will run

This 2D flow area connection will run

To edit the 2D flow area connection, select the element and then right-click and choose Edit Vertices from the displayed context menu (or press Function Key F4).

Edit Vertices from the displayed context menu

Next, grab the end vertex and resize the 2D connection so it covers the entire cell face.

resize the 2D connection

Then, right-click and choose Done from the context menu. Next, select the 2D connection and again right-click to display the context menu. Then, choose the Extract Weir Crest Geometry command.

Extract Weir Crest Geometry

The software will then display a dialog box, allowing you to select the terrain surface for extracting the weir crest geometry from.

extracting the weir crest geometry from

After clicking the [OK] button, the software will then update the weir crest geometry to occupy the cell face.

2D flow area connections

Now the model will run successfully.

About the Author Chris Maeder

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